6 Watson, Pectoral Girdle from the Lower Lias. 



line making at their symphysis a low obtuse ridge on the 

 lower surface, the scapulae are articulated with them so 

 that their lower surface is practically in the same general 

 plane as the coracoid of the same side, the clavicular arch 

 has its lateral wings tightly adpressed to the inner surfaces 

 of the scapulae, the middle of the arch forms a rather 

 broad plate, passing across between the widely-separated 

 anterior ends of the scapula;, and with its posterior border 

 some distance in front of the anterior processes of the 

 coracoids, except, possibly, for the median spine, which 

 may have reached them. The anterior edge of the arch 

 is directed slightly downwards, so that the arch, as a 

 whole, is in a somewhat different plane to the coracoids. 



The pectoral girdle just described is, on the whole, 

 the most primitive known in the Flesiosauria : it shews 

 that the primitive Sauropterygian had a T-shaped inter- 

 clavicle, with narrow clavicles joining it on both sides. 

 This T-shaped interclavicle seems to shew definitely that 

 the median bone of the Sauropterygian clavicular arch is 

 not a sternal element, for it is obviously similar to the 

 T-shaped interclavicle of the majority of the early reptiles. 



In correlation with the compression which the anterior 

 border of the girdle has to bear in consequence of the 

 forward thrust of the head of the humerus(see Andrews :io) 

 the clavicular arch is strengthened by the backward growth 

 of the clavicles along the posterior spine of the interclavicle, 

 with which they are strongly united. This enlargement of 

 the clavicles may also be associated with the development 

 of a powerful musculus claviculo-brachialis inserted on to 

 them and the lower and anterior part of the humerus. 

 Such a muscle may have played an important part in the 

 forward motion of the humerus, and have helped to turn it 

 into position for the stroke. The two small foramina 

 bounded by the clavicles and separated by the posterior 



